Add log levels, new log framework, verbosity

[jthomperoo]

Resolves #90

[codecov]

Codecov Report

Merging #93 into master will increase coverage by 0.9%.
The diff coverage is 86.04%.

@@            Coverage Diff            @@
##           master      #93     +/-   ##
=========================================
+ Coverage   95.74%   96.64%   +0.9%     
=========================================
  Files          10       10             
  Lines         329      358     +29     
=========================================
+ Hits          315      346     +31     
+ Misses          8        6      -2     
  Partials        6        6

Flag	Coverage Δ
#unittests	96.64% <86.04%> (+0.9%)	⬆️

Impacted Files	Coverage Δ
config/config.go	100% <ø> (ø)	⬆️
api/api.go	91.66% <0%> (+2.47%)	⬆️
scaler/scaler.go	100% <100%> (ø)	⬆️
evaluate/evaluate.go	88.23% <83.33%> (+4.9%)	⬆️
metric/metric.go	95.12% <85.71%> (+0.83%)	⬆️